change fix according to review

Author: Marco Gorelli

doc/source/whatsnew/v1.0.1.rst

@@ -15,7 +15,7 @@ including other versions of pandas.
 
 Bug fixes
 ~~~~~~~~~
-- bug ``Groupby.apply`` was returning ``TypeError`` if called with function which returned list (:issue:`31441`)
+- Bug in :meth:`GroupBy.apply` was raising ``TypeError`` if called with function which returned list (:issue:`31441`)
 
 Categorical
 ^^^^^^^^^^^

pandas/_libs/reduction.pyx

@@ -499,12 +499,10 @@ def apply_frame_axis0(object frame, object f, object names,
                 # `piece` might not have an index, could be e.g. an int
                 pass
 
-            if isinstance(piece, list):
-                piece = deepcopy(piece)
-            elif not is_scalar(piece):
+            if not is_scalar(piece):
                 # Need to copy data to avoid appending references
                 if hasattr(piece, "copy"):
-                    piece = piece.copy(deep="all")
+                    piece = deepcopy(piece)
                 else:
                     piece = copy(piece)
 

pandas/tests/groupby/test_apply.py

@@ -830,6 +830,7 @@ def test_apply_index_has_complex_internals(index):
 
 
 def test_apply_function_returns_list():
+    # GH 31441
     df = pd.DataFrame(["A", "A", "B", "B"], columns=["groups"])
     result = df.groupby("groups").apply(lambda x: x.index.to_list())
     expected = pd.Series([[0, 1], [2, 3]], index=pd.Index(["A", "B"], name="groups"))